High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
In compression ignition engines equipped with both VGT and EGR systems, optimal steady-state performance in terms of fuel economy and emissions is achieved by coordinating the operation of the two actuators. The steady-state performance of a compression ignition engine is directly related to the control system's ability to maintain the air/fuel ratio (AF) and the EGR fraction at desired values which depend upon engine speed and torque (as determined by the operator-requested fueling rate). Thus, in the engine control system the EGR fraction and AF ratio are the performance variables.
There are difficulties in regulating these performance variables to achieve optimum engine performance. In particular, the performance variables cannot be directly measured. In addition, there is significant interaction between the VGT and EGR actuators since both the VGT and EGR regulate gas flow from the exhaust manifold, and the exhaust gas directly affects the performance variables which are to be regulated. Consequently, there exists a need for a robust engine control strategy having stable regulation of the AF ratio and EGR fraction.